In the mid-1980's, high efficiency power converters became practical because of the introduction of new types of semiconductor switching devices, making possible higher switching frequencies, greater power efficiency, and smaller size and weight. Among other things, this has led to the incorporation of inverters, converters, switching regulators, and similar circuits into the power handling and distribution systems onboard satellites. A frequently used component in power circuits is the power Schottky rectifier diode (also referred to as a power Schottky diode). The power Schottky diode can be more efficient than the traditional PN-junction rectifier, having a lower forward voltage drop and a faster turn-off switching speed, because the forward conduction mode of the power Schottky diode involves majority carriers. For this reason, power Schottky diodes are now used frequently in power handling circuits in space applications, e.g., in satellites or space vehicles. Unfortunately, as discussed in greater detail below, previously known power Schottky diodes are vulnerable to SEB (single event burnout), which is not observable on Earth under normal conditions, but can be observed in the space environment where charged cosmic ray particles, e.g., heavy ions such as protons or atomic nuclei, can impinge upon the power Schottky diode, causing damage. There are believed to be hundreds or perhaps thousands of previously known power Schottky diodes now in power systems in space applications, which are believed to be vulnerable to SEB.
First, the construction of an exemplary, previously known power Schottky diode will be described. Then, a brief description of SEB will be provided.
FIGS. 1A-1B illustrate cross sections of an exemplary, previously known power Schottky diode. More specifically, FIG. 1A illustrates an overview of previously known power Schottky diode 100; the inset of FIG. 1A illustrates greater detail of certain features of diode 100 at an approximate, exemplary scale; and FIG. 1B illustrates greater detail of those certain features of diode 100 in an exaggerated scale so as to facilitate understanding. FIG. 2A, described in greater detail below, illustrates a plan view of certain components of an exemplary, previously known power Schottky diode, such as that illustrated in cross-section in FIGS. 1A-1B.
As illustrated in FIGS. 1A-1B, previously known power Schottky diode 100 includes substrate 110, active region 120, anode electrode 130 (which also may be referred to as a “top metal”), and cathode electrode 140 (which also may be referred to as a “back metal”). Active region 120 is disposed over a first surface of substrate 110, anode electrode 130 is disposed on active region 120, and cathode electrode 140 is disposed over a second surface of substrate 120 that is opposite to the first surface. In the previously known power Schottky diode 100 illustrated in FIGS. 1A-1B, substrate 110 is formed of silicon; anode electrode 130 includes aluminum and includes or is disposed on adhesion layer(s) 129b, such as titanium tungsten (TiW) for facilitating durable contact between anode electrode 130 and active region 120 and inhibiting “spiking” of metal into substrate 110 or into active region 120; and cathode electrode 140 includes a titanium layer followed by Au—Ge alloy or Ni/Au layered metal.
As perhaps best seen in FIG. 1B, active region 120 can include anode 121, semiconductor 122, and one or more oxide(s) 123. The portion of substrate 110 upon which active region 120 is disposed can be doped, e.g., N+ doped, so as to define a cathode contact, which can be at substantially the same voltage as is cathode electrode 140. Semiconductor 122 can be disposed over the first surface of substrate 110, and can include epitaxially deposited silicon (which also can be referred to as “epi”). Different regions of semiconductor 122 can be doped differently than one another. In the illustrated example, a first region 125 of semiconductor 122 is lightly doped N-type during epitaxial deposition, so as to define a cathode region; a second region 124 of semiconductor 122 is doped so as to define a guard ring, e.g., a P-type guard ring; and a third region 126 of semiconductor 122 is also lightly doped N-type as part of the cathode that becomes depleted under reverse bias.
In the previously known power Schottky diode 100 illustrated in FIGS. 1A-1B, anode 121 is disposed over semiconductor 122 and at least a portion of guard ring 124, and includes platinum silicide (PtSi2) or other suitable silicide, such as MoSi2 (molybdenum silicide), TiSi2 (titanium silicide), or others. Anode 121 can be at substantially the same voltage as is anode electrode 130, which is disposed over anode 121. For example, as is illustrated in FIG. 1B, anode 121 can be at a reverse voltage of −200 volts, and cathode (substrate) 110 can be at a voltage of 0 volts. A junction between anode 121 and semiconductor 122 can define a Schottky barrier in a manner well known in the art. The guard ring can reduce the electrical field gradient at the edge of the Schottky barrier in a manner well known in the art.
Oxide(s) 123 can include one or more discrete layers of oxide material. For example, oxide(s) 123 can include a first, relatively high quality and relatively thin oxide material 127 disposed over guard ring 124, and also can include a second, relatively low quality and relatively thick oxide material 128 disposed over the first oxide material. In the previously known power Schottky diode 100 illustrated in FIGS. 1A-1B, first oxide material 127 is formed of silicon dioxide (SiO2), and second oxide material 128 is formed of tetraethyl ortho silicate (TEOS). Additionally, metal layer 129a, such as platinum, followed by refractory metal 129b such as TiW, can be disposed over second oxide material 128.
As shown in FIGS. 1A-1B, previously known power Schottky diode 100 can have a lateral dimension of approximately 60 mils and a thickness of approximately 12 mils. The region generally designated (A), defined by a lateral dimension between the outer edge of the guard ring defined by region 124 and the inner edge of second oxide material 128, can have a lateral dimension of approximately 1.5 mils. The region generally designated (B), defined by a lateral dimension between the inner edge of second oxide material 128 and the inner edge of first oxide material 127, can have a lateral dimension of approximately 1 mils. The region generally designated (C), defined by a lateral dimension between the inner edge of first oxide material 127 and the inner edge of guard ring 124, can have a lateral dimension of approximately 1 mils.
As noted above, the exemplary power Schottky diode illustrated in FIGS. 1A-1B includes a Schottky barrier defined by a junction between anode 121 and region 126. A depletion layer 126 forms in N-type epitaxial region 125 in the reverse biased or off state. In forward operation, electrons (majority carriers) in N-type epitaxial region 125 are able to surmount the energy barrier that exists at the metal-semiconductor interface defined by the junction between 121, 126 and can flow into anode electrode 130 (top metal). Note that the conventionally defined current flow is from anode 121 to cathode (substrate) 110 in forward bias (and electron flow is from cathode to anode in forward bias). The relatively light doping and relatively thick epitaxial region 125 supports a relatively high breakdown voltage under reverse bias conditions. For example, an N-type epitaxial layer doping in region 125 of approximately 5×1014 cm−3 and a thickness of about 15 μm can be appropriate for a breakdown voltage of approximately 200 V. The breakdown voltage can be a function of the guard ring geometry and doping level, as is well known.
As is known in the art, a PtSi2 layer, such as used for anode 121 illustrated in FIGS. 1A-1B, can be formed by reacting a deposited Pt metal layer with Si in semiconductor 122 to form a PtSi2 layer. The process of this reaction can consume the Si surface and can drive surface contamination or oxide away from the interface. The PtSi2-to-N-type silicon barrier height is approximately 0.85 eV; different metal silicides with a similar barrier height can also be employed as anode 121.
As noted above, in addition to the PtSi2 anode 121, previously known power Schottky diode 100 can include P-type guard ring 124, which can be defined by doping a first region 124 of semiconductor 122 disposed about the periphery of the Schottky barrier. The guard ring 124 can terminate the edge of the metal-semiconductor Schottky junction where a crowding of the electric field lines under reverse bias occurs. Such a crowding can cause undesirable reverse leakage current and soft breakdown characteristics. Guard ring 124 can spread out the electric field at the edge and provide a suitable breakdown characteristic of a cylindrical PN junction. A typical guard ring of a power Schottky diode can be moderately doped P-type, e.g., with a boron concentration of about 5×1017 cm−3 to a depth of about 2 and can have a resistivity of about 0.05 Ω-cm and a sheet resistance of about 250 Ω/□.
FIG. 1C illustrates an exemplary circuit representation of an exemplary, previously known power Schottky diode. The circuit representation includes the main metal-semiconductor Schottky diode with a PN guard ring junction diode, of relatively smaller area, connected in parallel with the Schottky diode. The PN-junction does not turn on significantly in the forward direction because it is shunted by the Schottky barrier that has a significantly lower forward voltage drop. Therefore there is no significant minority carrier injection from the P-type guard ring into the N-type epitaxial semiconductor region under forward bias, and thus there is no significant minority carrier reverse recovery time when switching the diode off. Fast turn-off is an exemplary advantage of a power Schottky diode as compared with a conventional rectifier diode. For example, relatively fast turnoff can allow for relatively higher switching frequencies (trends are approaching 1 MHz), which in turn can allows for smaller transformer, energy storage inductors, and other magnetics, which can in turn lead to smaller weight.
Referring again to FIG. 1B, another feature of previously known power Schottky diode 100 includes the use of a relatively thin, relatively high quality oxide region 127 to laterally delineate the metal-semiconductor junction, e.g., an oxide material of approximately 200 nm thickness. Note that term “thin” here is used in the relative sense for power devices, whereas in the CMOS realm such thicknesses may be considered to be relatively thick. The relatively thin oxide 127 need not support the full applied reverse voltage, because it is substantially surrounded by anode electrode 130 and by heavily doped P-type guard ring silicon 124 that also can be at the same potential as anode electrode 130. A relatively thick, relatively low quality oxide region 128, such as tetraethylortho silicate (TEOS) oxide can be used to support the full reverse voltage as needed at the far edges of the diode. The top anode contact of the power Schottky diode, e.g., anode electrode 130, can be formed of relatively thick metal, e.g., aluminum (Al), which can provide for ruggedness and bondability. A different metal such as Au could also be used, and is sometimes employed, although much more costly. A refractory metal film or metal alloy film such as Mo, TiW, NiW etc. (singly or in multiple layers), e.g., layer(s) 129b illustrated in FIG. 1B, can also inhibit the Al from “spiking” into the PN junction and causing high leakage. These refractory films, e.g., layer(s) 129b also can aid in adherence of the thick top anode metal.
Contact can be made to the P-type guard ring 124 with the same PtSi2 metal 121 that forms the Schottky barrier on the N-epitaxial material 125. Beneficially, however, PtSi2 anode 121 forms a nearly ohmic contact to P-type silicon because the PtSi2 to P-type silicon barrier height is relatively low (approximately 0.25 eV). The back contact metallization, e.g., cathode electrode 140, as shown in FIG. 1A, but not visible in FIG. 1B, typically can include an evaporated Ti layer followed by Au—Ge alloy or Ni/Au layered metal. The typical area of the Schottky junction might be at least 50×50 mils (2500 sq. mils) for a relatively small diode. The guard ring is typically about 4 mils in peripheral extent around the Schottky contact, producing a PN junction area of about 400 square mils in this example, a relatively small percentage of the Schottky area. As noted above, FIG. 1A and its inset show a relatively correctly scaled cross sectional view of previously known power Schottky diode 100. The die itself can be about 60 mils square, while the Schottky barrier can be about 50 mils square. The edge termination and guard ring are shown approximately to scale in the inset of FIG. 1A.
In space satellites, the power supply is unavoidably exposed to relatively high energy cosmic rays. These rays can include relatively heavy, ionized particles (such as protons and atomic nuclei, which may be collectively referred to herein as “heavy ions”) travelling through space at relatively high speeds. Such particles travel from outside the solar system and their origin is unknown. The atmosphere surrounding the earth absorbs virtually all these particles so the terrestrial environment for a power Schottky diode is relatively benign. However in the space environment, these particles are capable of penetrating the skin of the satellite and the casing of the power supply, and can travel through the electronic components themselves, including any power Schottky diodes. If a particle of sufficient energy impinges upon the power Schottky diode while it is in its reverse blocking state, a single event burnout (SEB) can occur.
It is believed that in order for a SEB to occur in a power Schottky diode, a few conditions are simultaneously satisfied. For example, the power Schottky diode can be in its reverse blocking state at the instant that the particle is received; the particle can have a sufficiently high kinetic energy; and the particle can impinge upon the power Schottky diode near the guard ring. SEB tests have been performed on planar power Schottky diodes from many different manufactures. For example, SEB can be simulated in the laboratory using a synchrotron or cyclotron radiation facility to generate a beam of ionized particles. FIG. 2A illustrates a plan view of certain components of an exemplary, previously known power Schottky diode including a SEB damage site caused by a heavy ion strike, e.g., from cosmic rays or as simulated in the laboratory. In FIG. 2A, a top view of Schottky diode 100 is shown with all of the metallization and Schottky metal layers removed, showing the surface of the depletion region 126 of semiconductor 122. It is believed that SEB damage sites typically occur near or at the edge of the guard ring (e.g., ring 124) that is adjacent to the Schottky metal (e.g., anode 121, not shown in FIG. 2A). For example, FIGS. 2B and 2C illustrate images of exemplary, previously known power Schottky diodes including a SEB damage site caused by a heavy ion strike during testing in the laboratory of a power supply (FIG. 2B) or the diode alone (FIG. 2C). It respectively can be seen in FIGS. 2B and 2C that the damage sites 250, 250′ appear relatively similar to one another and are both relatively close to the edge of corresponding guard ring 224, 224′. Note that the scratches in FIG. 2C due to die extraction and handling are unrelated to the damage site.
The response of a targeted semiconductor device, e.g., power Schottky diode, can be characterized by its threshold linear energy transfer (LET) value, and its saturated cross section. Both of these parameters are a measure of the susceptibility of the device to heavy ions. For suitable use of a power Schottky diode in a space application, the burnout threshold LET can be relatively high, and the saturated cross section relatively low. For example, as the reverse bias on the power Schottky diode is increased, the threshold LET can become lower, and the saturated cross section can become higher. For suitable use of a power Schottky diode in a space application, the diode can be derated relative to the manufacturer's rating, which can be an undesirable compromise. The reliability of the power supply can be based on the waveform of the switching voltage; for example, if the voltage switching time is sufficiently short, the probability of a burnout can decrease concomitantly. As such, a voltage switching waveform having a DC reverse voltage or having a large reverse duty cycle is not believed to be feasible with previously known power Schottky diodes in space applications. This too can create an undesirable compromise for such devices.